Method of treating magnesium



' I q a a Patented Sept." 15', 1942 METHOD OF TBEATING MAGNESIUM Joseph- D. Hanawalt and Charles E. Nelson, Mu-

land, Micln, assignorl to.The Dow Chemical Company, Midland, Mich a. corporation of Michigan No Drawing. Application November 16, 1940,

Serial No. 365,986

7 Claims; ('01. 148-6) A a specimen of magnesium or alloy thereof for one minute in a 3 per cent aqueous sodium chloride solution held at 30 C. and then exposing the specimen to air for two minutes and repeating these steps in succession for several months. The average loss in weight of the specimen, expressed in milligrams per square centimeter per day after being subjected to the above treatment for from 1 to 4 months, is used as a measure oi corrodibility.

Magnesium and magnesium alloys in which the impurities, iron, nickel, copper, and silicon are present in relatively small amounts, the limiting amount of each depending upon the particular composition, have corrosion rates of not substantially more than 0.2 milligram per square centimeter per day. We have made a great number of magnesium alloys having such corrosion characteristics, the following being illustrative examples of compositions having a corrosion rate oi not substantially more than 0.2 milligram per square centimeter per day: (l) a magnesium base alloy containing as the alloying ingredient from 1.0 to'3.0 per. cent manganese and having .005 per cent of the impurity iron present, .002 per cent of the impurity nickel present, and .03 per cent each of the impurities copper and silicon present; (2) a magnesium base alloy containing as the alloying ingredients from 2.0 to 12.0 per cent aluminum and from .01

to 2.0 per cent manganese, and having .001 per cent of the impurity iron present, .0005 per cent of the impurity nickel present, and .03 per cent each of the impurities copper and silicon present; (3) a magnesium base alloy containing as the alloying ingredients from 2.0 to 8.0 per cent aluminum, .01 to 2.0 per cent manganese, 1.0 per cent or less of zinc and having .001 per cent each, of the impurities iron and nickel present, and .03 per cent of each of the impurities copper and silicon present; (4) a magnesium base alloy containing as the alloying ingredients from manganese, 3.0 per cent or more of zinc. and having .002 per cent 01. the impurity iron present, .001 per cent of the impurity nickel present, and 0.1 per cent of the impurities copper and silicon present; and (5) magnesium having .002 per cent 01 the impurity iron present, .0002 per cent of the impurity nickel present, .03 per cent of the impurity copper present, and .02 per cent of the impurity silicon present. The amounts of the impurities present in the illustrative compositions listed above are not to be construed as limiting amounts, since lesser, or somewhat greater amounts may be present. Alloys'such as those described above may be made in several ways,

as for example by melting pure distilled mag-- nesium in a crucible of graphite of high purity under a flux consisting of '70 per cent magnesium chloride and 30 per cent sodium chloride and adding the pure alloying ingredient or ingredients thereto.

The limiting amount of the impurity or impurities, as the case may be, that may be present in order to give an alloy that has a corrosion rate oi not substantially more than 0.2 milligram per square centimeter per day depends not only upon the particular composition or alloy but upon the number of the impurities that are present. While the corrosion rate above specified is very small compared to that of the known commercial magnesium or its alloys, once the limiting amount of impurity or impurities, as the case may be, is exceeded by only a small amount, in general the corrosion rate at once becomes that of ordinary commercial magnesium or its alloys.

While magnesium and alloys thereof in which the impurities are present in relatively small amounts are already very resistant to corrosion in comparison with commercial magnesium and its alloys, we have discovered that we can apply a coating thereto which is of a highly decorative nature, and, in addition, the coating renders the magnesium article or alloy thereof even more highly resistant to corrosion.

The invention, then, consists in the method hereinafter fully described and particularly pointed out in the claims.

We have discovered that magnesium and alloys thereof having a corrosion rate of not substantially more than 0.2 milligram per square centimeter per day may be coated with a hard, dense, highly corrosion resistant coating by subjecting them to the action of hot water which contains a suitable salt of the type normally 2.0 to 8.0 per cent aluminum, .01 to 2.0 per cent W highly corrosive to the commonly known mag nesium and its alloys. Further, we have discovered that the coating s'o produced may be dyed a wide variety of colors or, if desired, the

dye may be included in the coating bath prior to applying the coating, in which case a'dyed coating is produced.

Among the salts that we have found suitable to produce a coating -in accordance with the invention are the water-soluble chlorides, bromides, and iodides of the metals located above aluminum in the electromotive series of metals. While magnesium and its alloys having acorrosoin rate of not substantially more than 0.2 milligram per square centimeter per day are readily coated in-accordance with the present invention, commercial magnesium and its alloys having higher rates of corrosion are-not coated when subjected to treatment according to our discovery, but, instead, are rapidly corroded away. For example-if an article of magnesium or its alloys having a corrosion rate of not substantially more than 0.2 milligram per square centimeter per day is immersed in a boiling aqueous bath containing 3 per cent sodium chloride, 9. very hard, dense, glossy coating is produced on the article in a short time and continued action of the bath serves only to increase the thickness of the coating.

subjecting an article of commercial magnesium or its alloys to identical treatment causes a nonadherent coating of magnesium hydroxide to form at the surface of the article and, as the reaction continues, the magnesium article corrodes rapidly away without the formation of a thick protective coating. Such reaction continues until the article completely disintegrates one or more of the aforementioned salts. The

coating produced is hard and highly resistant to corrosion, particularly in contact with dissimilar metals, besides being highly decorative in nature.

The temperature of the coating bath may be held between about 85, to 120 C., and preferably at about 100 C. It is usually more convenient to operate the bath at atmospheric pressure, although satisfactory coatings may also be produced at pressures above atmospheric. Illustrative of suitable pressures are from 5 to 15 atmospheres, although higher or lower pressures may be employed. In case pressure is used the abovenoted temperature range is satisfactory. The article being coated should be subjected to the action of the bath for between about to hours, depending upon the thickness of the coat ing desired, the temperature, and composition of the bath. At 100 C. and at atmospheric pressure a satisfactory coating can be produced in 10 hours using a 0.1 per cent sodium chloride solution as the treating bath.

As aforementioned, the salts to employ for the purpose at hand are the chlorides, bromides, and

iodides of the metals located above aluminum in the electromotive series of metals. The concenby subjecting the coated article to the action of an aqueous solution of a suitable dye at a temperature between about and C. for about hour or more.. Suitable dyes to employ are those not discharged by an alkali and not decomposed at the temperature employed and soluble in water. The term "not discharged by an alkali" used herein and in the appended claims means that the presence or an alkali neither prevents t the dye from coloring the article nor aifects the fastness of the color produced by the dye. Examples of such dyes are Alizarin Cyanine, Janus Green, Safranine, Gentian Violet, and the like.-

The amount of dye to employ should be between about 0.1 and 0.5 per cent or more by weight of the dyeing solution, depending upon the shade desired. In general, the more concentrated the dye in the solution, the deeper will be the shade of color produced. If desired, a dye having the characteristics aforementioned may be added to the coating bath, in which case, as the coating is produced, it is dyed or colored the desired shade. By the foregoing method coatings that 'are hard, thick, highly res stant to corrosion, and strongly adherent may be produced having practically any desired shadeof color.

The present application is a continuation-inpart of application Serial No. 253,343, filed January 28, 1939. l

We claim: 1. A method of coating a light metal article comprising subjecting an article of magnesium andits alloys having a corrosion rate of not substantially more'than 0.2 milligram -per square centimeter per day to the action of an aqueous solution containing a salt selected from the group consisting of chlorides, bromides, and iodides of the metalslocated above aluminum in the elecand its alloys having a corrosion rate of not substantially more than 0.2 milligram per square centimeter per day 'to the action of an aqueous solution containing a water-soluble salt selected from the group consisting of the chlorides, bromides, and iodides of the metals located above aluminum'in the electromotive series of metals at a temperature between about 85 and C. and at atmospheric pressure for from /2 to 30 hours, whereby a coating is produced on said from the group consisting of the chlorides, bromides, and iodides oi the metals located above aluminum in the electromotive series of metals at a temperature between about 85 and 120 C.

tration to employ to produce a satisfactory coatand at pressureabove atmospheric for V2 to 30 hours, whereby a coating is produced on said magnesium article.

4, A method of coating a light metal article com-prising subjecting an article of magnesium and its alloys having a corrosion rate of not more than 0.2 milligram per square centimeter per day to the action of an aqueous solution containing between about 0.01 to per cent of a soluble salt selected from the group consisting oi! the bromides, chlorides, and iodides of the metals located above aluminum in the electromotive series of metals at a temperature of from 85 to 120 C. at atmospheric pressure for from /2 to hours.

5. A method of producing a coating on a light metal article comprising subjecting an article of magnesium and its alloys having a corrosion rate of not substantially more than 0.2 milligram per square centimeter per day to the action of an aqueous solution containing about 0.1 per cent of sodium chloride at a temperature of 100 C. and at atmospheric pressure for from /2 to 30 hours.

6. In a method of coating and dyeing a light metal article comprising subjecting an article of magnesium and its alloys having a corrosion rate of not substantially more than 0.2 milligram per square centimeter per day to the action of an aqueous solution containing a. salt selected from the group consisting of the chlorides, bromides, and iodides of the metals located above aluminum in the electromotive series of metals at a temperature .between.85 and 120 C., and thereafter dyeing the article so treated by immersing it in a solution of a dye not discharged by an alkali maintained at a temperature between and C.

7. A method of producing a dyed coating on a a light metal article comprising subjecting magneslum and its alloys having a corrosion rate of not substantially more than 0.2 milligram per square centimeter per day to the action of an aqueous solution at a temperature between 85 and C. said solution containing a soluble salt selected from the group consisting of chlorides, bromides, and iodides of the metals located above aluminum in the electromotive series of metals,

7 and a dye soluble therein and not discharged by an alkali, whereby a colored coating is produced on said article.

JOSEPH D. HANAWALT. CHARLES E. NELSON. 

